1. Voltaic (Galvanic)Cells
The energy released in a spontaneous redox reaction is used to perform electrical work.
Voltaic or galvanic cells are devices in which electron transfer occurs via an external circuit.
Voltaic cells are spontaneous.
If a strip of Zn is placed in a solution of CuSO4, Cu is deposited on the Zn and the Zn dissolves by forming Zn2+.

2. Zn is spontaneously oxidized to Zn2+ by Cu2+.
The Cu2+ is spontaneously reduced to Cu0 by Zn.
The entire process is spontaneous.

4. Voltaic cells consist of
Anode: Zn(s)  Zn2+(aq) + 2e-
Cathode: Cu2+(aq) + 2e-  Cu(s)
Salt bridge (used to complete the electrical circuit): cations move from anode to cathode, anions move from cathode to anode. Necessary to balance charge.
The two solid metals are the electrodes (cathode and anode).
Anode (-) = oxidation Cathode (+) = reduction

5. “Rules” of voltaic cells:
As oxidation occurs, Zn is converted to Zn2+ and 2e-. The electrons flow towards the cathode (through the wire) where they are used in the reduction reaction.
We expect the Zn electrode to lose mass and the Cu electrode to gain mass.
“Rules” of voltaic cells:
1. At the anode electrons are products. (Oxidation)
2. At the cathode electrons are reactants. (Reduction)
3. Electrons cannot swim.

6. Electrons flow from the anode to the cathode.
Therefore, the anode is negative and the cathode is positive.
Electrons cannot flow through the solution; they have to be transported through an external wire. (Rule 3.)

8. Anions and cations move through a porous barrier or salt bridge.
Cations move into the cathodic compartment to neutralize the excess negatively charged ions (Cathode: Cu2+ + 2e-  Cu, so the counterion of Cu is in excess).
Anions move into the anodic compartment to neutralize the excess Zn2+ ions formed by oxidation.

9. A Molecular View of Oxidation-Reduction Processes
Consider the spontaneous redox reaction between Zn(s) and Cu2+(aq).
During the reaction, Zn(s) is oxidized to Zn2+(aq) and Cu2+(aq) is reduced to Cu(s).
On the atomic level, a Cu2+(aq) ion comes into contact with a Zn(s) atom on the surface of the electrode.
Two electrons are directly transferred from the Zn(s) (forming Zn2+(aq)) to the Cu2+(aq) (forming Cu(s)).
In a voltaic cell, the electrons are transferred through the circuit from Zn to Cu.

12. Cell EMF The flow of electrons from anode to cathode is spontaneous.
Electrons flow from anode to cathode because the cathode has a lower electrical potential energy than the anode.
Potential difference: difference in electrical potential. Measured in volts.
One volt is the potential difference required to impart one joule of energy to a charge of one coulomb:

13. Electromotive force (emf) is the force required to push electrons through the external circuit.
Cell potential: Ecell is the emf of a cell.
For 1M solutions at 25 C (standard conditions), the standard emf (standard cell potential) is called Ecell.

14. Standard Reduction (Half-Cell) Potentials
Convenient tabulation of electrochemical data.
Standard reduction potentials, Ered are measured relative to the standard hydrogen electrode (SHE).
Oxidation Potentials are obtained by reversing a reduction half-reaction (sign of E° is changed)

17. The SHE is the cathode. It consists of a Pt electrode in a tube placed in 1 M H+ solution. H2 is bubbled through the tube.
For the SHE, we assign
2H+(aq, 1M) + 2e-  H2(g, 1 atm)
Ered of zero.
The emf of a cell can be calculated from standard reduction potentials: